This amended subproject proposes to document the fundamental processes of hair cell regeneration following ototoxic destruction (using gentamicin) of the vestibular inner ear organs in a mammalian model (chinchilla). The specific aims of this subproject are: (1) Documentation of ototoxically mediated histological changes in the vestibular sensory epithelium, including both qualitative and quantitative measurements of hair cell recovery and reinnervation; (2) Documentation of the effects of local administration of brain derived neurotropic factor in the recovery of vestibular sensory epithelium following gentamicin ototoxicity, and (3) Investigation of the temporal course of changes of vestibular reflex associated with vestibular hair cell destruction and regeneration. There are three experiments (with three sub-experiments in each of the experiments) to be conducted using chinchillas Experiment 1 is to determine the time course and level of vestibular and organ recovery following gentamicin (GM) ototoxicity by studying (a) the quantitative aspects of hair cell repair and regeneration, (b) the qualitative morphological characteristics of hair cell regeneration, (b) the qualitative morphological characteristics of hair cell regeneration and reinnervation using light, electron,, and scanning microscopy techniques, and (c) reinnervation using quantitative immunohistochemical techniques. Experiment 2 is to investigate the effects of brain derived neurotropic factor (BDNF) by (a) characterizing the effect of BDNF on recovery of the vestibular neuroepithelium after GM lesion, (b) analyzing the dose response effect on BDNF-assisted neuroepithelium recovery, (c) studying the time course of BDNF effect on regeneration, and (d) determining the extent of cell proliferation. Experiment 3 is designed to determine the time and temporal course of vestibular reflex recovery following GM ototoxicity by measuring the vestibulo-ocular reflex (VOR), visual vestibulo-ocular reflex (VVOR), and optokinetic reflex (OKR) using the magnetic search coil, and the vestibulocolic reflex (OKR) using the magnetic search coil, and the vestibulospinal reflex (VSR) using electromyography. These reflex functions will be correlated with histological indices of repair or restoration or peripheral morphology, the number of hair cells and the establishment of reinnervation patterns.